Transmitters measure process variables representative of a process controlled by remote devices and communicate transmitter outputs representing the process variables to controllers over two wire circuits. The transmitters are typically mounted in a field area where current and voltage levels are limited to provide intrinsic safety. The transmitter output is scaled by user definable parameters such as span, zero and damping. Span and zero adjustments allow the user to reference the measurement range extremes of the transmitter to specific transmitter output levels, thereby setting the range of desired outputs. Damping affects the response time of the transmitter to changes in the process variable. The scaled transmitter output wire is sent over the two wire circuit to the controller.
Controllers, typically located in a control room, combine the transmitter output with signals representing other process variables to generate command output signals. Command output signals are typically sent over a separate pair of wires to remote devices, such as a valve, which control the process according to the command output. In certain applications, controllers select the most appropriate set of instructions for process control from multiple sets of instructions, depending on the process to be controlled and the accuracy required.
In other applications, controllers sense several transmitter outputs representing process variables to determine the command output for the remote device. Typically, a separate transmitter senses each process variable. The transmitters send a signal representative of the sensed process variable to the controller over a two wire circuit, the controller determines the command output and controls the remote device, such as a valve position, pump speed, thermostat setting, etc.
One limitation of the prior art arrangement is that the transmitter, remote device and controller, components in the feedback loop, must all operate continuously for control of the process. Another limitation is the amount of interconnecting cabling connecting the feedback loop components. The controller is typically far from the process in a control room, while the remote device and the transmitter are usually in the field and physically proximate to each other and the process. Installation and maintenance complexity is another limitation, since each cable may require installation of an intrinsic safety barrier device at the interface between the control room and the field devices. In cases where multiple process variables are used by the controller, cabling is required between each transmitter and the controller. Feedback loop reliability is a fourth limitation, since failure of one of several interconnections adversely affects process control.
To reduce these limitations, a process variable transmitter provides a control output directly, thereby bypassing the controller itself. Outputs representative of other process variables are communicated to the transmitter rather than the controller. The transmitter can still communicate with the controller over a common two-wire link, but the link can be broken without interrupting control. Process control reliability and response time is enhanced and control is realized with fewer communication exchanges while installation complexity, maintenance complexity and cost decreases.